A number of prior art processes have been proposed for the preparation of alkylene carbonates, especially ethylene and propylene carbonates. Such methods consist of, e.g., direct preparation from an olefin, catalytic conversion of epoxides with carbon dioxide at high temperatures, and reacting halohydrins with carbon dioxide and alkaline carbonates or bicarbonates.
The present invention is directed to an improved process for the liquid phase preparation of an alkylene carbonate, such as propylene carbonate, in high yield and avoiding problems associated with the prior art processes. More particularly, the present process relates to the synthesis of alkylene carbonates by reacting the corresponding alkylene iodohydrin in a solvent reaction medium with carbon dioxide under elevated temperature and pressure conditions in the presence of oxygen or an oxygen-containing gas and an effective amount of a catalytic mixture of (1) an iodide of the metals selected from group IA, IB, IIA, IIB and VIII metals of the Periodic Table of Elements and (2) a carbonate of the metals selected from group IB, IIA and IIB metals of the Periodic Table of Elements, at a pH value of between about 3 and 10.
U.S. Pat. No. 2,766,258 discloses a method for the preparation of ethylene and propylene carbonate by reacting a chlorohydrin with a stoichiometric amount of a carbonate, bicarbonate or sesquicarbonate under a pressure of carbon dioxide.
U.S. Pat. No. 2,773,070 describes a process for the preparation of alkylene carbonates by treating at elevated temperature and pressure conditions an alkylene oxide with a molar excess of carbon dioxide in the presence of a quaternary ammonium halide.
U.S. Pat. No. 2,784,201 discloses a process for making alkylene carbonates by heating a stoichiometric amount of an alkali metal lower alkyl carbonate with a lower alpha, beta-alkylene chlorohydrin and distilling an alkanol from the reaction mixture.
U.S. Pat. No. 2,873,282 discloses the preparation of alkylene carbonates by reacting an alkylene oxide with carbon dioxide in the presence of a quaternary ammonium hydroxide, carbonate, bicarbonate or ion-exchange resin having quaternary ammonium groups.
U.S. Pat. No. 3,923,842 describes a method for the preparation of cyclic carbonate esters by reacting the corresponding vicinal halohydrin with carbon dioxide in the presence of a stoichiometric amount of an amine compound, such as diethylamine.
The direct preparation of alkylene carbonates including by-product halohydrins from olefins is described in U.S. Pat. No. 4,009,183 employing carbon dioxide and oxygen in the presence of a catalytic system consisting of (a) iodine, metal iodides including alkali and alkaline earth metal iodides and iodohydrines of the olefin and (b) an oxygen carrier selected from manganese dioxide, nitrites, nitrates, nitrogen oxides and a cobalt complex. In addition to substantial amounts of halohydrins being produced as by-product, the carboxylation reaction rate is slow.
The alkylene carbonate products obtained by the process of this invention have known commercial applications, particularly as reactants for the preparation of alkylene oxides such as propylene oxide by decomposition of the carbonate and as organic polymer fiber-forming resin solvents.
A particular feature of the invention in addition to the catalyst system, is the employment of alkylene iodohydrins in conjunction with oxygen in the reaction to oxidize the hydrogen iodide, produced by the reaction with carbon dioxide, to iodine and water thus shifting the equilibrium reaction to the right to completion in favor of the alkylene carbonate in accordance with the following: ##STR2## wherein R and R' are as hereinafter defined. The catalyst system in addition to providing a faster rate of reaction and selectivity to the alkylene carbonate than prior art processes also provides, with the use of the metal carbonate salt, the necessary buffer to control the pH of reaction medium within the desired range.